This happened to me recently twice to be exact, both timers didnt go off but the alarm is on the screen and yet no sound at all. Im using iphone 13 pro with the newest ios update. I tried the alarm too and its the same no sound but it will show it on the screen.
Also my bf have been having this issue for 2 years now with his iphone 12 at first I couldn't believe it and said hes just a heavy sleeper thats why he couldn't hear his alarms at all lol
Now he switched to the newest iphone model and he hasn't encountered it yet.